One can try to react against social norms by, for example, refusing to conform to sexual norms of morality, but this reaction is in and of itself an acknowledgement of the pervasiveness of the social ideal that Heidegger called inauthentic. Perhaps the classic example of this is the teenager that reacts against his or her parents by doing everything the opposite of what they advise, and thus only shows how deeply the parent's rules have become internalized in his or her consciousness. For the postmodernist, one cannot help but be engaged with the world. Unlike existentialists like Heidegger that saw people as chronically disengaged with reality, postmodernist thought sees engagement as a given. One cannot live outside of culture and language. For the postmodernist, by rejecting the authentic self, a greater measure of freedom is actually achieved because the individual can both eschew either total obedience to a constructed social ideal, and escape the constant trap of internalizing the ideal as a way of reacting to that ideal. This is why, for postmodernism, irony and satire rather than dropping out of society are the primary weapons with which to fight back against societal constraints. To take another example, imagine a woman who wishes to reject her culture's notions of femininity....

To do so, she could try to be more masculine, but this would only confirm the masculine/feminine binary of selfhood and thus confirm her engagement with false social ideas about gender. Instead, by assuming female traits or male traits with irony, and by knowing that she can pick and choose which characteristics and traits she wished to embrace, she would evolve into a more subversive and potentially challenging figure. This would not necessarily be a 'truer' self, for there is no true self, but it would be a potentially freer self.
Postmodern notions that explode notions of the authentic self seem more fruitful than merely reacting against notions of selfhood provided by social institutions with an insistence upon a pure 'authentic self.' To follow Heidegger's mode of resistance would seem to require some form of social withdrawal, to avoid the dangers of inauthenticity. Accepting that the notion of an authentic self is a fiction frees the individual from the burden of having to find a fixed answer to the question of 'who am I?' Finding one's self can seem just as much of a burden to finding an answer to the totality of existence a la Hegel, or constructing the end of history, a la Marx. Postmodernism's humor and playfulness may partly come from its techniques of pastiche and parody, but may also lie in the fact that it does not take itself or 'the self' too seriously in a refreshing fashion, in contrast to other philosophical schools.

Works Cited

Hornsby, Roy. "What Heidegger Means by Being-in-the-World." Oct 2002. 7 Aug 2007. http://royby.com/philosophy/pages/dasein.html